1. Technical Field
The present invention generally relates to an optical system; and more particularly to an system for monitoring the wavelength or some other optical parameter of an optical signal.
2. Description of Related Art
FIG. 1 shows a wavelength monitoring device 10 that is known in the art, constructed using an optical fiber 11 having a fiber core 11a, a cladding 11b surrounding the fiber core 11a and a blazed fiber Bragg grating 12 written in the fiber core 11a, and used in conjunction with a detector array 14, as shown and described in U.S. Pat. No. 6,002,822, issued to Lucent Technologies, Inc., which is hereby incorporated by reference in its entirety. (See also U.S. Pat. No. 5,337,382 which discloses a 125 micron optical fiber having Bragg gratings written therein at an oblique angle, which are known in the art as blazed gratings, which is also incorporated by reference herein.)
In FIG. 1, the blazed fiber grating 12 is used to decouple light generally indicated as L from the fiber core 1 a into the cladding 11b with the angle of departure from the fiber core 11a dependent on the wavelength of the light L. The decoupled light LD is reflected through a member 16. Using direct detection or with directing optics, the light L coupled into the cladding 11b can be detected using the detector array 14. As shown, the blazed fiber grating 12 serves as a wavelength dependent dispersive element and the detector array 14 as a spatial filter which gives a direct correlation between individual detector elements and a specific wavelength range. FIG. 1(a) shows the waveform of the detector array 14.
However, this technique has several drawbacks which limit its ultimate performance. When properly written, the blazed fiber grating will be able to create a uniform dispersion of the wavelengths covered by the grating element, but the ultimate resolution depends on the number and spacing of the detector array elements. If a full C-band (37 nanometers) device is desired, then a typical 256 element array will give a resolution of 145 picometers. If more resolution is desired, then an array with additional detectors (such as 512 elements) can be used; however, these devices can be quite expensive and the number of inoperable pixels dramatically increases as the total elements increase.